A heat dome, characterized by extreme heat and humidity, is expected to cover most of the United States and affect more than 220 million people. Temperatures are forecast to exceed 90 degrees, and for those living in the Northeast and Mid-Atlantic,

A punishing heat wave in the Southwest is set to spread east in the coming days, with temperatures over 90 degrees Fahrenheit expected in the Northeast and possible record highs in parts of New England later this week.

Humidity will soon surge to extreme levels across 40 states. Find out where there will be excessive heat and humidity — and how humidity patterns are changing.